Many structures are proposed that have forming methods and devices utilizing forming rolls to re-form round tubes of circular cross section to square tubes of square or rectangular cross section as the primary means for manufacturing square tubes.
For example, in describing the structure which displays the concept for the most general forming method (refer to the disposition of forming rolls in FIG. 6), there are utilized multiple so-called four-direction roll forming stands composed of one pair of upper-lower forming rolls and one pair of left-right forming rolls for which the rotation axes are disposed identically to the raw tube cross sectional, straightening of the raw tube portions is by pressing from four directions the rolls against the raw tube locations that correspond to the side portions of the final product, and allowing plastic deforming of the round cross section shape to a square or rectangular cross section shape.
In addition, there is a limit for the forming amount of any single stage of four-direction roll forming stand, so to reduce the number of forming rolls expected for control and equipment costs, generally, there are disposed 3˜4 stages of the above described forming stands along the axis direction of the raw tube, and there is caused successive deforming of the raw tube cross section shape.
On the other hand, in Japanese Laid-Open Patent Application No. 2000-301233, another roll forming device and forming method are disclosed as a means of re-forming for square tubes. With this forming means (see FIG. 7), the roll rotation axis of an upper-lower forming roll pair and the roll rotation axis of a left-right forming roll pair are disposed at differing raw tube cross sections. Accordingly, no reciprocal interference is present for any roll position adjustment, and, in addition, even with forming rolls having a single curvature in the rotation axis direction, it is possible to support differing product dimensions by utilizing such position adjustment, and there is easy adaptation to automated and NC processes.
Furthermore, for the purpose of simplifying forming devices, there are also offered devices (see FIG. 8) that cause plastic deforming of a round cross section shape by using multiple forming rolls having forming surfaces of a V-shaped concave portion only in two-directions substitutionally for four-direction roll stands, but there are easily generated problems for product surface damage due to excessive roll surface speed differences and problems for product shape symmetry, so this is limited to small-sized products where the forming rolls are sufficiently larger than the raw tube outside diameter, and it is not a general-purpose method. In addition, similarly to four-direction roll stands, there is little joint use of rolls for differing product dimensions.    Patent Citation 2: Japanese Laid-Open Patent Application No. H5-212440    Patent Citation 3: Japanese Laid-Open Patent Application No. H6-262253
Through the results of earnest investigation of former roll forming structures, the inventors recognized that adopting of shapes with a single arc having a fixed radius (R) in the rotation axis direction became a causal factor not only for problems related to dimensional accuracy, such as irregularities in the curvature of corner portions and degenerated flatness of side portions for square tubes following forming, but also for inviting insufficiencies of rigidity for square tube products due to excessive deforming in corner portions (see “a” of FIG. 4A and “A” portion of FIG. 4B) and side portions adjacent to corner portions (hereafter referred to as “shoulder portions”; see “b” of FIG. 4A and “B” of FIG. 4B), and that these problems easily generated destruction, etc., for these same locations.
In addition, the forming means disclosed in Japanese Laid-Open Patent Application No. 2000-301233 was developed as an effort to reliably apply multi-use implementation of rolls, but the inventors recognized that the previously described problems were not essentially eliminated because the curvature of the roll caliber in each roll was a single curvature or a straight line shape.
Furthermore, when considering forming raw tubes of various cross section curvatures using rolls with which each has a single caliber curvature, for example, when the curvature radius of the roll caliber is set to enable use in forming of raw tubes having a large cross section curvature radius, the curvature radius of the roll caliber becomes excessive for a raw tube having a small cross section curvature radius.
Accordingly, at use of the described roll caliber for a raw tube having small cross section curvature radius, when the forming amount is excessively large at a single stage due to the difference of those curvatures, indentation is easily generated in the side portions of the product and the flatness of the product is adversely affected.
To alleviate or eliminate this problem, it is necessary to divide the dimensions range for all products into multiple stages and to prepare each roll caliber corresponding to each raw tube dimension range. Described differently, when the products dimension range is wide, it is necessary to prepare rolls and forming stages of correspondingly greater number. This effect increases equipment costs, and effect of roll multi-use implementation becomes limited.
On the other hand, while the means of forming by establishing multiple stages of upper-lower forming roll pairs and left-right forming roll pairs is general-purpose, from the perspective of reducing equipment costs, there is strongly sought a means of reducing to the greatest extent the number of these forming stages. Moreover, it becomes necessary for equipment design to consider easy facilitation of preservation and maintenance control for the equipment.